In recent years, an optical filter which transmits light in a visible wavelength range but blocks light in a near-infrared wavelength range has been used for various applications.
For example, in an imaging device such as a digital still camera, a digital video camera, or a cellphone camera using a solid-state image sensor (such as CCD or CMOS), or a display device such as an automatic exposure meter using a light-receiving element, an optical filter is used for obtaining good color reproducibility. Spectral sensitivity of the solid-state image sensor or the light-receiving element is in a wavelength range ranging from an ultraviolet wavelength range to a near-infrared wavelength range, and on the other hand, human visibility is only in a visible wavelength range. In order to make the spectral sensitivity of the solid-state image sensor or the light-receiving element close to human visibility, an optical filter is disposed on a subject side of the solid-state image sensor.
Various types of such optical filters have been proposed. For example, there can be cited a reflection-type filter having a dielectric multilayer film which is formed by alternately stacking dielectric thin films with different refractive indices on a single side or both sides of a transparent substrate, and reflects light to be blocked by utilizing interference of light. In the filter having the dielectric multilayer film, optical characteristics thereof sometimes change, depending on an incident angle of light. For this reason, if such a filter is used, the spectral sensitivity of the solid-state image sensor may be influenced by the incident angle.
On the contrary, JP-A-2008-181028 and JP-A-2008-051985 describe, as optical filters which are less influenced by an incident angle of light with a wavelength of 600 to 800 nm, an absorption-type filter having an absorbing layer containing an absorbing dye in a transparent resin, and a filter formed by combining a dielectric multilayer film and an absorbing layer. In the filter having the absorbing layer, a change in optical characteristics caused by an incident angle of light is small, so that it is possible to reduce the influence of the incident angle of light with the wavelength of 600 to 800 nm, with respect to spectral sensitivity of a solid-state image sensor.
Further, JP-A-2013-190553 describes an optical filter having an absorbing layer containing a compound which absorbs light with a wavelength of 380 to 450 nm, which can reduce incidence angle dependence of the light with the wavelength of 380 to 450 nm.
Meanwhile, due to progress of high performance of the solid-state image sensor, the optical filter which has a wavelength with transmittance of 50% in 400 nm or more, and can increase average transmittance from a wavelength with transmittance of about 10% to a wavelength with transmittance of about 80% (to provide a steep change of transmittance) has been demanded. The optical filter described in JP-A-2013-190553 has not sufficiently satisfied these demands.